The Special Investigations Unit says the death of a 63-year-old Summer Beaver Reserve man was unrelated to his arrest earlier this month.
Provincial police arrested Albert Moonias on the morning of March 15 and put him in a cell at the Sioux Lookout detachment.
The SIU says Moonias was later found to be in medical distress and was transported to Sioux Lookout Hospital where he was pronounced dead.
A post mortem determined that Moonias had died as a result of a heart attack.
The police watchdog agency says there was no evidence of trauma to suggest that he was involved in a fight or restrained.
Director Ian Scott says the SIU has concluded Moonias died as a result of a condition unrelated to any actions by the police and has closed the file on the death.